High speed electronic digital computers of the type produced by Cray Research, Inc., the assignee hereof, typically require short length electrical connections between various electrical cards and chips. Shorter length electrical connections introduce fewer timing problems between signals within the high speed digital computer. Timing problems between electrical signals can result from electrical signals passing through different lengths of wire. The shorter length contacts also provide better electrical performance when compared to a longer contact. A longer contact has more inductance and capacitance when compared to a shorter contact. More importantly, different lengths of wire will produce different amounts of impedance through various lengths of wire. Differences in impedance include different capacitances and inductances. At various frequencies, the different impedances may introduce undesirable elements to the signals passing through the wires. It is desirable to have short and uniform length contact paths to lower impedance effects.
The general trend in electronic packaging has been to pack more electronics, such as transistors, onto a single substrate. Some microprocessors, for example, include 10,000 transistors in a 1.5 inch square substrate. One die on a substrate or module is called a single chip module (SCM). Placing multiple dice onto one substrate produces multi-chip modules (MCMs). Placing more electronic components onto a single substrate produces shorter length electrical connections between components, which is desirable from the standpoint of designing a computer. However, such dense electronic packaging gives rise to a host of other problems. One such other problem is that each module, whether its a MCM or SCM, has a high number of tightly spaced electrical connection points or input/output pads (I/O pads) for signals input to the module or for signals output from the module. In a computer system of any sort, various modules or substrates have to be interconnected to produce a fully functioning computer. Good, reliable electrical connections between selected I/O pads on one module or substrate and selected I/O pads on another module are required to build a dependable computer system. The electrical connections need to provide low electrical resistance to produce a reliable connection. All modules or substrates are now designed with the interconnection of the final product in mind, so that the I/O pads for one module correspond to the I/O pads for another module. As a result, an apparatus and method for making many electrical connections is needed. There is also a constant push toward smaller footprint modules with increased numbers of I/O pads. Consequently, there is also a need for an apparatus with a larger number of tightly spaced I/O pads.
Electrical connections between a module and a printed circuit board can be done in a number of ways. The attachment methods can be categorized into two methods. A module can be either hard attached or removably attached. Hard attachment means that the pins or pads of a memory chip, multiple chip module ("MCM") or single chip module ("SCM") are soldered directly to a printed circuit board ("PCB"). Removable attachment means there is some form of interconnection that allows two attached components to be separated. Both the hard attached modules and the removably attached modules have problems. Hard attachment has a disadvantage in that the electrical component module may have a different coefficient of thermal expansion ("CTE") than the printed circuit board to which it attaches. The coefficient of thermal expansion for silicon is lower than the coefficient of thermal expansion for a ceramic substrate, which is lower than the CTE of the PCB. When the printed circuit board is heated, the ceramic or silicon substrate does not expand as much as the printed circuit board. The pins located at the outside corners usually are stressed and fatigued the most and therefore fail first.
Demateable coupling of a ceramic or silicon electrical component and printed circuit board typically is not plagued with connection failures due to different coefficients of thermal expansion between the electrical components and the printed circuit board to which they are attached. However, demateable couplings typically have other problems. One of the larger problems with demateable couplings is compliancy. Lack of compliancy is a problem when an individual electrical component is coupled with another individual electrical component or with a printed circuit board. The interconnect must be compliant or allow for some different heights between the interconnection sites in order to be successfully attached. Lack of compliancy is also a problem when there are multiple electrical components to be attached to a single printed circuit board.
Making a direct electrical contact between a high number of non-compliant I/O pads on an individual electrical component and a high number of non-compliant I/O pads on another substrate or printed circuit board is nearly impossible.
Making a connection between electrical components and a printed circuit board having connection sites for multiple components requires compliancy or an allowance for different height components if all components are to remain the same total height for proper thermal management, i.e., contact to a cold plate. In other words, for a multi-site type of attachment, the compliancy must accommodate not only differences in height within the component but also must accommodate differences in height between components.
Also, it is difficult to uniformly distribute a proper amount of force over a high number of pads so that each I/O pad will form a reliable electrical connection with a corresponding I/O pad due to planarity. In addition, if the modules are brought together for the purposes of testing, all the electrical connections at each I/O pad would have to be made reliably for each test and would have to be made so that no significant wear was produced on the I/O pads of the module under test.
There are many apparatus and methods that provide for a compliant interconnection between an electrical component and a printed circuit board. Two more common ways for making the interconnections include use of woven wire contacts, and use of elastomeric sheets with embedded wires. A woven wire contact is an interconnect that includes a column of woven electrically conductive material. The electrically conductive material is a woven like steel wool. The column of woven material is compressed between a contact pad on the two mated devices, such as a contact pad on a substrate and a contact pad on a PCB. The use of woven material columns provides interconnection to contacts having a fairly wide range of heights when compared to the use of an elastomeric sheet filled with wires. However, the woven columns cannot be closely spaced, although they accommodate a wide range of height. When the I/O pads are closely spaced on the electrical components, the columns of woven conductive material are not available in a small enough dimension to provide an independent contact to each I/O pad or pin from the electrical component. If an independent connection is not made to each I/O pad, or if shorting is created, the electrical component is not capable of being used.
Another method and apparatus that has been used to connect contacts between a first component and a second component is an elastomeric sheet that contains conductive wire or fibers. There are many manufactures of this type of material. One such elastomeric sheet is approximately 2 mils to 3 mils in thickness and contains small wires which provide a conductive path through the elastomeric sheet. The elastomeric sheet is merely sandwiched between the I/O pads of the electrical component and the printed circuit board to provide an interconnection. The pads of the electrical component are aligned with the pads of the printed circuit board so that the electrical connections can be made. The elastomeric sheets can accommodate a higher pitch than the woven wire contacts, however, when compared to the woven wire contact material, the compliancy or the amount of height difference that can be accommodated is lower. The sheet is only 2-3 mils thick to start with so the maximum possible compliancy will be less than 2-3 mils whereas the woven wire compliancy may be up to 6 mils.
With respect to the use of woven columns of conductive material, a structure for accommodating a pitch of 40 mils or less is not available. In addition, the force that must be applied to connect an electrical component and a printed circuit board using woven columns are relatively large. A high force will be required in order to compress the high number of individual columns of steel wool-like material. For an interconnection with 1600 contacts, it is not uncommon to place 300 pounds of force onto the components being interconnected.
Placing such a high force on such a small substrate requires a fairly substantial mechanical device for applying the high force. Using such a large mechanical device is not conducive to a compact computer design. In supercomputers, there are many such interconnections since there are many processor units that must be interconnected with other substrates and there are also many memory units that also have to be mated with other components. Placing bulky mechanical devices inside a computer frame for each of the many interconnections that is required takes up space and makes cooling difficult. In summary, woven wire contacts can accommodate non-planarity of the chips, but large forces are necessary to accomplish this task. In addition, woven wire contact products are not available for low pitch products.
Using a sheet of elastomeric material with conductive wires located within the elastomeric material also has problems. The amount of compliance or accommodation of non-planarity is low. Therefore, the substrates that are going to be interconnected using an elastomeric material must be fairly planar. In addition, high forces are needed to produce the interconnection and obtain the compliance between the two contact pads. The electrical resistance when making connections with the elastomeric sheets is typically very high. Another problem is shorting or open connections. When the density of the interconnects gets high or, saying the same thing, the pitch between the interconnects gets low, the conductive fibers or res in the elastomeric sheet may go from one contact to another undesired contact, hereby producing a short. It may be impossible with low pitch I/O to obtain pad sizes large enough to insure no opens without also causing shorting.
As mentioned above, the elastomeric sheet interconnect has low compliance. The amount of non-planarity accommodated with one elastomeric material is approximately 2 mils which means that all the electrical contact pads on both substrates have to be within a 2 mil tolerance total. Such close tolerances increase manufacturing costs or simply are not available.
Elastomeric materials may also be subject to creep at high temperatures and very high stiffness (lack of compliance) at very low temperatures.
Another problem associated with existing interconnects is that the inductance, capacitance, and resistance between the contacts of existing connectors and the I/O pads on a module or chip will not allow for a high frequency operation of 100 MHZ and beyond. Present compliant interconnect systems do not offer high frequency performance at the I/O pad pitches or spacings which will be required for future computer systems. The current techniques for producing compliant interconnects do not employ the large area processing (LAP) techniques that result in low-cost fabrication.